Orthogonal Frequency Division Multiplexing (OFDM) is one of the Multi-Carrier Modulation (MCM) technologies. OFDM divides a frequency channel or frequency carrier into multiple orthogonal sub-channels each associated with a frequency subcarrier and allows for transmission of sub-data streams in parallel at low rates as opposed to a single data stream over a single frequency carrier at a much higher rate. The low-rate sub-data streams each modulate one of the sub-carriers. At the receiving end, the received orthogonal subcarriers each carrying a sub-data stream can be separated from one another using appropriate techniques to reduce inter-channel interference (ICI) between the sub-channels. Because the bandwidth of each sub-channel is smaller than the bandwidth of the channel, fading in each sub-channel generally can be considered flat across the frequency bandwidth of that sub-channel. As a result, inter symbol interference may be eliminated. In addition, because the bandwidth of each sub-channel is only a small portion of the bandwidth of the original channel, channel equalization also becomes easier.
The Long Term Evolution (LTE) project is directed to developing wireless communications standards as an evolution from 3G technology. The project began at the 2004 3rd Generation Partnership Project (3GPP) Conference held in Toronto. LTE uses OFDM and MIMO (Multiple-Input Multiple-Output) technologies. With a 20 MHz spectral bandwidth, LTE can provide a downlink peak rate of 326 Mbit/s and an uplink peak rate of 86 Mbit/s. LTE can improve user experience at cell edges, increase cell capacities, and reduce system latencies.
An LTE network may include a number of cells, each corresponding to a geographical area. Within each cell, communication terminals such as mobile phones or, more generally, user equipment (UE), access network services such as phone services or Internet services, data streaming, etc., through an interface station such as a base station, which is also referred to as eNode B in LTE terminology. An LTE system can implement either Time Division Duplex (TDD) or Frequency Division Duplex (FDD). In TDD, communications in two opposite directions between the base station and mobile phones occur in the same frequency band but different time slots. In FDD, communications in two opposite directions between the base station and mobile phones occur at the same time but in different frequency bands.
In a MIMO configuration, a transmitter such as a base station may transmit signals from multiple transmitting antennas, and a receiver such as UE may receive multiple transmitted signals at multiple receiving antennas. The base station may employ different transmission schemes with a MIMO configuration. For example, in a multi-stream MIMO scheme such as spatial multiplexing, the base station transmits different codewords from the different transmitting antennas. In a single-stream MIMO scheme such as transmit diversity, the base station transmits the same codeword from all transmitting antennas. Parallel transmission of different codewords from transmitting antennas provides higher throughput but requires better channel condition. If channel condition is not satisfactory, attempting to use spatial multiplexing may result in even lower throughput because a high pack loss rate may require repeated transmissions of the same packets.
In order for the base station to determine whether it should transmit multiple codewords in parallel or use transmit diversity to transmit the same codeword, UE must determine and report a Rank Indication (RI) to the base station. RI is an indication of the number of transmission layers or data streams the UE can distinguish. For example, an RI value of 1 indicates that the UE can only receive one stream at a time, meaning that the base station can only send one codeword at a time from the multiple transmitting antennas. An RI value of 2 suggests that the UE can receive two streams, allowing the base station to transmit two codewords in parallel from the transmitting antennas.
Conventionally, RI is determined by computing the rank of a channel matrix, which reflects the condition of the channels between the multiple transmitting antennas and receiving antennas. However, RI determined through such conventional method often does not accurately reflect the channel condition. For example, under many circumstances, with relatively strong correlation between the channels, the computation of the channel matrix rank results in a higher RI value, indicating that the channel environment is suitable for multi-stream transmission; yet noise and interference may be so high, or the channel environment so complicated (as in high frequency-selective fading or high speed movements), that a multi-stream MIMO transmission link may result in an even lower throughput than a single-stream diversity transmission link. Thus, under these circumstances, determining RI by computing the rank of the channel matrix may not produce reliable result but requires complex computation.
U.S. Patent Application Publication No. 2011/01051137(A1) discloses an example of a conventional method for a rank indication parameter detection, as well as methods and apparatuses for signaling rank indication and precoding matrix indications.
Therefore, there is a need for methods and apparatus that conveniently determine a rank of a MIMO channel to more accurately indicate whether the current channel environment allows a multi-stream MIMO transmission to improve throughput.